1. Field of the Invention
The present invention relates to an image heating apparatus which is suitable if it is applied to a heating fixing apparatus mounted in an image forming apparatus such as an electrophotographic printer, an electrostatic recording printer, a copying machine or the like, and more particularly to the image heating apparatus which uses a flexible sleeve.
2. Related Background Art
Conventionally, many of electrophotographic copying machines, printers and the like adopt a heating roller fixing system of a contact heating type which is excellent in thermal efficiency and safety or a film heating system of an energy saving type, as a heating fixing means.
The heating fixing apparatus which adopts the heating roller fixing system basically consists of a heating roller (called fixing roller hereinafter) acting as a rotation member for heating which contains a halogen heater, and an elastic pressure roller (called pressure roller hereinafter) acting as a rotation member for pressing which is in pressure contact with the fixing roller. Such a pair of the rollers is rotated, a recording material (a transfer material sheet, an electrostatic recording material, an electrofax sheet, a print sheet, or the like) acting as a material to be heated on which an unfixed image (a toner image) has been formed and borne is introduced to a fixing nip part acting as a pressure-contact nip part for the pair of the rollers, and the introduced recording material is thus nipped and conveyed, whereby the toner image is heat-pressed and fixed to the surface of the recording material as a permanent fixed image by heat from the fixing roller and pressure force of the fixing nip part.
On the other hand, as proposed in, e.g., Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075 to 4-44083, 4-204980 to 4-204984, and the like, the heating fixing apparatus which adopts the film heating system closely contacts a heat-resistive film (called fixing film or flexible sleeve hereinafter) acting as a rotation member for heating with a heating member (called heating body hereinafter) such as a fixedly disposed ceramic heater or the like by a rotation pressure member (called pressure roller or pressure member hereinafter), thereby slidingly rotating the fixing film. Then, a recording material on which a toner image has been formed and borne is introduced to a fixing nip part acting as a pressure-contact nip part which is structured so that the fixing film is placed between the heating body and the pressure roller, and the introduced recording material is conveyed together with the fixing film, whereby the toner image is heat-pressed and fixed to the surface of the recording material as a permanent fixed image by heat applied from the heating body through the fixing film and pressure force of the fixing nip part.
The heating fixing apparatus which adopts the film heating system can use, as the heating body, a small thermal-capacity linear heating body such as the ceramic heater or the like, and also use a small thermal-capacity thin film as the fixing film, whereby it is possible to save power and shorten a waiting time (i.e., achieve quick start). Incidentally, a method of providing a drive roller on the interior surface of the fixing film and a method of using the pressure roller as a drive roller and thus driving the fixing film by the frictional force between the drive roller and the pressure roller are known as a fixing film driving system to be used in the heating fixing apparatus which adopts the film heating system. In recent years, pressure roller driving system is frequently used because the number of parts is low and the cost is low.
FIG. 9 is a view schematically showing the structural model of one example of the heating fixing apparatus which adopts the pressure roller driving system and the film heating system.
In FIG. 9, numeral 30 denotes a heating assembly and numeral 32 denotes an elastic pressure roller which acts as the pressure member. The heating assembly 30 and the elastic pressure roller 32 which are disposed in parallel above and below are pressure-contacted with each other to form a fixing nip part N.
The heating assembly 30 is the assembly which consists of a heater 33 acting as the heating member (heating body), a film guide 35 acting as the guide member supporting the heater 33, a cylindrical fixing film 31 containing the film guide 35 and acting as the flexible rotation body internally contacted with the heater 33, a flange member 36 supporting the fixing film 31 by its both ends and fit to the film guide 35, and the like.
The heater 33 is the oblong and thin ceramic heater of which the longitudinal length expands along the direction perpendicular to the conveying direction of a recording material P and the heat capacity is entirely small, and the heater 33 receives power supply and thus generates heat.
The film guide 35 is the gutterlike oblong member of which the cross section is substantially a semicircular arc and the longitudinal side extends in the direction perpendicular to the conveying direction of the recording material P, and, for example, the film guide 35 is made of phenolic thermosetting resin. The heater 33 is fit into a heater fit groove which is formed longitudinally on the approximately central part on the under surface of the film guide 35 and thus fixedly supported.
The cylindrical fixing film 31 is loosely fit outwardly to the film guide 35 into which the heater 33 has been fit.
The flange member 36 includes a collar washer part 36a which catches the end part of the cylindrical fixing film 31 and regulates the movement of the fixing film toward its axis line direction, and a fixing film sliding part 36b which is substantially a circular arc and is fit to the inside of the end of the cylindrical fixing film 31 to support the fixing film end. The flange member 36 is fit to both ends of the film guide 35 and thus settled.
The elastic pressure roller 32 is rotatably bearing-supported between the side covers (not shown) of the heating fixing apparatus, the heating assembly 30 is disposed in parallel above the elastic pressure roller 32 with the heater 33 side downward, the heating assembly 30 and the elastic pressure roller 32 are pressed into each other by a not-shown pressure means against the elasticity of the pressure roller 32, and the heater 33 and the pressure roller 32 are thus pressure-contacted with each other so that the fixing film 31 is placed between the heater 33 and the pressure roller 32, whereby the fixing nip part N which acts as the pressure-contact nip part of a predetermined width is formed due to the elastic deformation of the pressure roller 32.
The elastic pressure roller 32 is rotatively driven counterclockwise as indicated by the arrow by a not-shown driving means. By rotatively driving the pressure roller 32, rotative force is applied to the fixing film 31 in the fixing nip part N due to the frictional force between the pressure roller 32 and the exterior surface of the fixing film 31. Then, the interior surface of the fixing film 31 is rotated clockwise as indicated by the arrow around the periphery of the film guide 35 at a periphery speed substantially corresponding to that of the pressure roller 32 as the interior surface of the fixing film 31 is in close contact with and slides along the lower surface of the heater 33 in the fixing nip part N (pressure roller driving system).
The movement of the rotating fixing film 31 in its axis line direction (longitudinal direction) is regulated by the collar washer part 36a of the flange member 36, and the inside of the end of the fixing film 31 is supported and rotatively guided by the fixing film sliding part 36b of the flange member 36.
Then, in a state that the fixing film 31 is rotatively driven by the pressure roller 32 and the temperature thereof has reached a predetermined temperature due to electrification to the heater 33, when the recording material P on which an unfixed toner image T has been formed and borne is introduced from a not-shown image forming part to the position between the fixing film 31 and the pressure roller 32 in the fixing nip part N, the recording material P passes the fixing nip part N together with the fixing film 31 in the state that the recording material P overlaps and is in close contact with the exterior surface of the fixing film 31.
While the recording material P is passing the fixing nip part N, the thermal (or heat) energy of the heater 33 is applied to the recording material P through the fixing film 31, whereby the unfixed toner image T on the recording material P is subjected to a heating melt fixing process. After then, the recording material P which passed the fixing nip part N is separated from the surface of the fixing film 31 at a separation point A and then discharged.
In regard to the image forming apparatus such as the electrophotographic printer or the like which uses the above heating fixing apparatus of the film heating system, increase in print speed is demanded according to improvement of image quality in recent years. Even if a passing time of the recording material in the fixing nip part N is shortened due to the increase in print speed, it is necessary to lower a melting point of the toner as well as improvements such as increase in fixing temperature, enlargement of the fixing nip part N, and increase in heat conduction of the materials of the heater substrate and the fixing film in order to maintain the fixability of the toner image T to the recording material P equivalent to the conventional level.
From the viewpoint of maintaining the fixability corresponding to high-speed print, a significant effect can be achieved by the combination of the increase in thermal energy supply amount to the recording material P per unit time due to the increase in fixing temperature, the enlargement of the fixing nip part and the like and the lowering of the melting point due to increase in, e.g., low molecular weight component of the toner. However, if the fixing temperature (i.e., a target temperature of the heater) is set to be high, also the temperature of the film guide 35 becomes high, and the heat of the film guide 35 is conducted to the fixing film 31 too. As a result, the toner temperature on the fixing film 31 and the recording material P at the separation point A thus becomes high. Therefore, cohesion of the toner T on the fixing film 31 and the recording material P at the separation point A becomes lower than adhesion between the toner T and the fixing film 31, whereby it becomes easy to cause a so-called hot offset T′ that the toner remains on the surface of the fixing film 31 after the film was separated from the recording material P.